Hybrid rocket motors or “hybrid motors” have recently been given greater attention in the space community. Hybrid motors use reactants of different physical phase states, usually a solid fuel such as rubber and a liquid or gaseous oxidizer, such as nitrous oxide. Hybrid motors do not generally deliver the performance of liquid motors. However, hybrid motors are safer and simpler to build and to operate. Hybrid motors can have good performance but often have problems maintaining the proper oxidizer-to-fuel or “O/F” ratio over the duration of the burn. Hybrid motors also tend to be physically long along the rocket motor axis for the same reasons. Hybrid motors can have complicated systems for introducing the gaseous oxidizer portion at different positions lengthwise in the fuel section.
Hybrid solid fuel bodies are generally two-dimensional shapes extruded into the third dimension. A simple example is a thick-walled tube extruded along the length of the tube. Such a tube is characterized as having a center axial flow channel, usually called a “port.” The oxidizer is injected through an intake opening and into the solid fuel body and out through a nozzle as exhaust. The fuel is ignited by an igniter positioned proximal to where the oxidizer first contacts the fuel near the intake. The solid fuel bodies generally have a center elongated flow channel through which the oxidizer flows after ignition for ablating the fuel on the side walls of the center elongated flow channel. The fuel is burned on the internal surface effectively ablating, or “regressing,” the solid fuel interior wall. As the fuel is burned, and port diameter increases, the combustion becomes oxidizer rich. If oxidizer-rich burning occurs, additional oxidizer must be carried, and the efficiency of the system is typically poor. Complex fuel grain shapes are sometimes used to increase the amount of surface area in the elongated center flow channel, but sometimes at the risk of an unsupported section of fuel breaking off and plugging the nozzle, causing a catastrophic failure of the hybrid motor. As the fuel burns through the elongated center flow channel, the oxidizer burns the inside of the channel. The growing diameter of the elongated center flow channel changes the ratio between the oxidizer flowing in the channel and the exposed burning fuel on the side walls of the elongated center flow channel. The hybrid rocket motor suffers from changing the oxidizer-to-fuel ratio. The oxidizer-to-fuel ratio becomes oxidizer rich and thereby wastes available oxidizer that could otherwise be used for burning more of the fuel.
Another problem associated with hybrid motors at least for use in launch vehicles, is low regression rates, typically one-third of that of composite solid propellants. Regression rate is the depth-wise rate at which the fuel is removed from the surface where burning occurs. This is a factor in the development of rocket engine thrust. A great amount of research has gone into replacing the solid rocket boosters on the Space Shuttle with hybrid motors only to show that hybrids suffer from low regression rates, which may make replacing large solid motors very difficult.
A problem associated with the fabrication of hybrid motors where paraffin wax is the fuel, is approximately 15% shrinkage as the liquid paraffin wax cools to a solid. Fuel grains, which are cast, can develop voids or bubbles. One method of alleviating this problem is to spin or otherwise rotate the fuel grain in a lathe-like apparatus so that the fuel is forced against the motor casing and a port naturally forms along the motor axis. This technique can limit the minimum port diameter and lends itself to a relatively simple round-shaped port, which is usually not compatible with the use of additives, particularly those with different densities than paraffin wax. Additives of a higher density than paraffin wax would be flung toward the motor casing rather than being evenly distributed throughout the fuel grain.